to do a base fog on film i enlarge, i use a basic ordinary sytrofoam cup. I test to see how much it will fog when i hold it over the lense and the enlarger turned on. I do one second tests like a test strip when printing. I go for just a hint of fog. If someone has thrown out my high tech piece of equipment, i resort to letting our not so safe safe lights add a bit of fog. with them it takes a count of 11 to do what I want on a particular enlarger. Then wonders of wonders i juxst sue the same chemistyr for the people working with paper developing to do my postives, then the larger negatives. You can check out my van dyke and cyanotype in the technical gallery to see how successful I have been.